Electronic parking brakes, also known as electrical or automatic parking brakes, are increasingly replacing mechanically operated parking brakes in motor vehicles. The use of electronic parking brake systems eliminates the parking brake lever, which is usually rather large, in the passenger compartment, thus increasing available space. Furthermore, electronic parking brake systems provide greater operating ease because the operator does not have to apply relatively high levels of force in order to apply or release the brakes. In addition, various driving situations such as pulling away on a hill or releasing the brake when first starting from parking are more easily performed electronically and/or also automatically by use of such systems.
Although providing numerous advantages, electronic parking brake systems are prone to several problems. One problem relates to flowing of adhesive used to bond brake components to one another. And, depending upon the configuration of the vehicle brake system, another problem relates to the system exhibiting excessive noise or vibration. In some vehicles, the electronic parking brake system uses the same brake components, i.e. brake pads and brake shims, as the hydraulic brake system of the vehicle. As is now common, brake shims are typically used in vehicle brake systems to reduce brake noise. Brake shims are typically thin layers of rubber or metal positioned between the brake pads and rotors. Brake shims are typically bonded or affixed to brake pads by the use of adhesives, either exclusively or in conjunction with mechanical fasteners. In order to counter excessive noise and vibration, e.g. “brake squeal,” resulting from brake application such as to slow or stop the vehicle when moving, vibration absorbing pressure sensitive adhesives such as certain silicone and acrylic pressure sensitive adhesives have been used for bonding brake shims to brake pads. However, these adhesives have been found to flow, particularly during application of the brakes when parking. Flowing of silicone and acrylic adhesives occurs as a result of the high pressures applied for long time periods when a parking brake is applied. Furthermore, the tendency for an adhesive to flow is increased when the brakes are at high temperatures.
In view of these and other problems associated with currently known silicone and acrylic pressure sensitive adhesives, formulators have investigated other types of adhesives. Phenolic adhesives, which do not flow under relatively high temperature and pressure, are now typically used. A problem however with phenolic adhesives is that they do not have sufficient noise and vibration damping properties. Thus, brake systems using phenolic adhesives to bond brake shims to brake pads, typically exhibit brake squeal or emit other undesirable noises when slowing or stopping.
Accordingly, a need exists for an adhesive that exhibits sufficient bonding properties so that it can be used to bond brake shims to brake components, which does not flow in parking brake applications, and which sufficiently dampens vibration when slowing or stopping to thereby counter noise and brake squeal.
Another disadvantage associated with many adhesives currently used to bond brake shims to brake pads is that application of the adhesive requires costly equipment. Adhesive layer thickness must be controlled within relatively high tolerances. And, typically, such adhesives are difficult to handle due to their viscoelastic properties which significantly promote the sound deadening characteristics of the adhesives.